(1) Field of the Invention
The present invention relates to a solid oxide fuel cell generation system and start up method thereof.
(2) Description of Related Art
In the solid oxide fuel cell, it is necessary to heat up a whole to a predetermined temperature capable of generating power at a time of starting. Further, a hydrogen necessary for generating power is supplied by reforming a city gas, a propane gas or the like corresponding to a raw fuel, and there are an internal reforming method in which the raw fuel is directly supplied to an anode side so as to be reformed in an inner portion of a power generation cell, and an external reforming method in which a reformer is provided in an outer portion of the power generation cell, and a gas reformed by the reformer is supplied to the power generation cell. Particularly, in the case of the external reforming method, it is necessary to heat up the reformer to a temperature capable of carrying out a reforming reaction, in the same manner as the power generation cell.
In the conventional power generation cell heating method, there is employed a method of heating by an electric heater attached around the power generation cell, a method of supplying an air heated to a high temperature by setting a heating apparatus in an air piping supplying to a cathode, or the like.
In this case, if an oxidizing gas such as the air or the like comes into contact with the anode side, the anode is oxidized, and a conductivity and an electrode activity are lost. Accordingly, there are various devices in preventing the oxidizing gas such as an oxygen or the like from flowing in the anode, for example, by circulating a hydrogen gas as a reducing gas to the anode side at a time of starting.
For example, in patent document 1 (JP-A-11-162492), there is disclosed a method of heating while keeping a reducing atmosphere by supplying an incomplete combustion gas to the anode side, as one example of the internal reforming method.
At a time of starting the reformer, the reformer is heated by circulating the high-temperature gas used for heating the power generation cell mentioned above to the combustion gas side of the reformer. It is necessary to supply the reducing gas such as a nitrogen, the hydrogen or the like corresponding to an inert gas to the fuel gas side flow path in the light of preventing a reforming catalyst from being deteriorated, in the same manner as the power generation cell. Further, a water vapor which is necessary for a reforming reaction in the reformer is supplied by evaporating a stored pure water by an evaporator, from the start to the power generation.
In this case, for example, in patent document 2 (JP-A-2004-247247), there is disclosed a means for utilizing a water generated within the anode on the basis of an electrochemical reaction for reforming, by recycling a reacted fuel gas in an outlet of the anode to the reformer in accordance with a fuel recycle system, with regard to reservation of the water vapor, during the power generation. Accordingly, the pure water and the evaporator are not necessary conceptually.
However, in accordance with the start up method of the fuel cell described in the patent document 1, the anode side is not in the reducing atmosphere until the cell temperature reaches 400° C., and the combustion gas of the oxidizing atmosphere is supplied thereto. Accordingly, there is considered that a reliability of the anode can be further improved, if it is possible to heat in the reducing atmosphere from an initial stage of the start. Further, there is a case that an oxidation resistance can be kept only at a further lower temperature, due to a material of the fuel cell, a difference of a manufacturing process or the like.
It is preferable to set an air ratio to be equal to or less than 1 for generating the combustion gas of the reducing atmosphere, however, the combustion temperature comes to 2000° C. at an adiabatic flame temperature in the vicinity of 1. Accordingly, it is not preferable in the light of a strength of the fuel cell to directly make the room-temperature fuel cell expose to the high-temperature combustion gas, because a heat stress is generated due to a great temperature difference. If the air ratio is lowered for holding down the temperature of the combustion gas, there is generated such a problem as a generation of soot and an instability of combustion.
Further, there is a method of adjusting the temperature by mixing the nitrogen of the inert gas with the reducing combustion gas having an air ratio equal to or less than 1, for example, 0.9, however, an auxiliary facility for supplying the nitrogen is necessary, and a system is complicated. In other words, there has not been developed a means supplying the reducing gas to the anode without using the inert gas such as the nitrogen or the like, and without generating any great temperature difference in the inner portion of the fuel cell.
The patent document 1 describes the start up method of the internal reforming method, and does not describe the power generation system of the external reforming method. In the external reforming method, it is necessary to secure the water vapor required for reforming before the power generation, and accordingly the pure water and the evaporator thereof have been conventionally necessary.
In accordance with the patent document 2, the pure water and the evaporator are not necessary conceptually during the power generation in accordance with the fuel recycle method, however, the patent document 2 does not disclose a system structure and a start up method which are necessary for achieving both the heating of the power generation system, and the reservation of the water vapor and the control of the reforming rate, from the start to the power generation.